Growing evidence suggests that the discrete coupling, cellular architecture and heterogeneities at multiple spatial scales play a major role in the initiation, maintenance and termination of arrhythmia under certain diseased conditions and particularly during aging. Consequently, the development of appropriate and effective antiarrhythmic pharmacological or biological therapies requires a better understanding of the factors that modulate impulse propagation in a heterogeneous substrate in critical regimes where conduction can fail. In particular, this proposal seeks to answer the following fundamental questions: 1) Is it necessary to model diseased tissue substrates with micro-heterogeneity using micro-structural models or can the dynamics of interest can be captured using continuous models, as is now the practice?; 2) How and why are impulse conduction and reentry dynamics altered in the context of structural heart disease?; and 3) Does tailoring the basic electrical properties of introduced/altered non- myocytes within diseased cardiac tissue could improve conduction and prevent reentry induction? We plan to use the predictive properties of the validated model to inform future experiments and inspire a rational approach for designing novel antiarrhythmic therapies. The aims are 1) To develop and refine a parameter estimation technique to derive the microscale electrical properties of a monolayer of neonatal rat cells and to use the combined computational/experimental approach to investigate the effects of cell orientations and patterns of cell coupling on microscopic impulse conduction in a geometrically controlled structural setting. 2) To use a combined theoretical/experimental framework to develop a diseased model of cardiac tissue to investigate the roles of conduction barriers and heterogeneous tissue structure on impulse conduction and reentry dynamics in a geometrically controlled structural setting. 3) To use the framework in Aim 1 to study the effect of non-myocytes on cardiac impulse conduction and reentry dynamics in a geometrically controlled structural setting, and to use the models to develop therapeutic strategies to facilitate conduction. The methods proposed here will allow us to create computer models that have one-to-one correspondence with the monolayers with regard to cell shape and cell-to-cell connectivity. Once validated, the models will be extended to adult architectures and diseased tissue for which it is not currently possible to develop engineered tissue. Ultimately these models will be used as a testbed to design new experiments to study the role of heterogeneity on arrhythmia and develop novel approaches for reengineering the arrhythmogenic substrate either through novel cell therapies or new drugs.